Fairly simple really, I just want people to be able to drag an image onto another image, position it where they want by dragging it around, and then when they're happy with it, click a button to save it (on my server) as a file (preferably a PDF or PNG).

This may sound like a simple, beginner's question, but I am not asking HOW to accomplish this, just what the best technology currently is (JS, HTML5, something else, maybe some existing libraries ala jQuery). I have been focusing on back-end stuff for a few years and am TOTALLY out of touch with what is the best way to do this. I don't want to use Flash, I know that.

I'm looking for a method that is both cross-platform, and easy enough to learn, or preferably if there are good libraries I can adapt easily.